1. Field of the Invention
The present invention relates to holography and, more particularly, to a simple optical apparatus for simultaneously measuring in real time the spectral and angular selectivities, the wavelength of maximum diffraction efficiency, and the direction of the grating vector of a reflection-type holographic optical element.
2. Description of the Related Art
A holographic optical element (HOE) is a hologram that is used to control transmitted light beams, rather than to display images. The angular and spectral selectivities of the HOE represent the deviation of incident angle satisfying the Bragg condition and the deviation of wavelength in the incident angle, respectively. Angular and spectral selectivities are measured, since reflection-type HOEs are used primarily as filters, beam combiner, and image display screens.
A conventional method for measuring angular and spectral selectivities involves measuring the diffraction efficiency for each wavelength or each incident angle by means of an instrument such as a monochrometer, a photodetector, or spectrophotometer. Conventional methods for measuring the angular and spectral selectivities:
(a) require a high-precision rotational or translational stage to control the angle of incidence of the beam incident on the HOE; PA1 (b) require one detector to measure the spectral selectivity and another detector to measure the angular selectivity, because the position of the detector is fixed when the spectral selectivity is measured; PA1 (c) cannot be used to tune an HOE in real-time, since the HOE must be rotated or repositioned after a measurement at a certain position or wavelength, both because the position or wavelength of an incident beam must be varied to measure the angular or spectral selectivity, respectively, and because whether HOE was tuned should be measured while swelling or shrinking its activity by wetting or heating for tuning HOE; PA1 (d) are prone to error, since the beam on which measurements are made is not a collimated beam; and PA1 (e) cannot readily be employed when several grating vectors are present in one HOE.